This invention relates to improved magnetic coating media for flexible (floppy) disk products of the type used in magnetic recording, particularly digital recording.
Some of the problems faced by manufacturers of flexible disks used in magnetic recording are: (1) to provide a coating which is opaque to the light, usually infrared signal, which is transmitted selectively through apertures in the disk and used as a control means when the disk is in use on a disk drive, (2) to provide a level of conductivity in the disk's coating for the elimination of static charge, and (3) to provide a tough, abrasion and wear resistant coating. It has been customary to use a substantial quantity of carbon black to achieve such opacity and conductivity characteristics.
However, use of carbon black in the coating to achieve the desired opacity and conductivity had certain undesirable features. Carbon black has a very high binder demand. High levels of carbon black will tend to weaken and soften the coating. In some binder systems, this creates a need for another non-magnetic filler as an abrasive additive to toughen the coating surface and act as a head cleaning agent when the disk is operating in the drive. Also, use of carbon black and other non-magnetic additives limits the level of magnetic pigment which may be used in the coating.
Initially, Applicant directed his efforts to finding a means to improve the recording performance of the flexible disk coatings by reducing the coating thickness required and thus increasing the resolution of the recording signal. By decreasing the thickness of the coating and bringing the recording head closer to that portion of the magnetic media coating which is closest to the substrate on which the media is carried, the high frequency (2F) signal can be increased and the low frequency (1F) signal is directly decreased. However, as will be disclosed below, his investigation led to substantial and unforeseen improvements in the manufacture of flexible disk recording media.
In heretofore, unrelated developments, some inventors have suggested that certain benefits may be derived from the use of FeO in magnetic coating compositions. For example, in U.S. Pat. No. 4,276,183 to Wilson, the use of 1-4 weight percent FeO is disclosed as being related to the achievement of superior orientation ratios, squareness, and the improvement of chemical stability in a wide variety of recording materials. Although it is mentioned that his coating formulations can be used in disks, it does not appear that he actually used them in disks or discovered any particular advantages of the use of FeO in disks.
German Offenlegeningschrift No. 1,907,236, cited in U.S. Pat. No. 3,897,354 to Woditsch, discloses the desirability of using some FeO to ensure uniform quality and constant resonance loss in the magnetic tape. However, it is also stated that it is difficult to repeatedly obtain the FeO content necessary for such properties.
Another publication which relates to the art discussed below includes U.S. Pat. No. 4,430,782 which describes the technology of burnishing flexible disks.
The above-identified references are necessarily discussed with a hindsight review of the invention. The prior art disclosed was turned up only in a post-invention review of the art and it is not intended to suggest that these references would have been consulted by one of ordinary skill faced with the problems to which the inventor has directed his efforts.